Grozny, August 9 (RFE/RL) -- Chechen forces appear to control much of the capital, Grozny, after four days of fierce fighting and the Organization for Security and Cooperation in Europe has appealed for an immediate ceasefire.

Federal troops have been unable to recapture key areas of the city center taken by the separatists, a senior Russian officer told Interfax.

Rebel spokesman Movladi Udugov says the rebels now control the main administrative center of the Moscow-backed government and the Interior ministry building.

Eyewitness accounts appear to confirm this, although some reports say fighting is still continuing around the buildings, which have sustained heavy damage.

An AP reporter says the rebels are using captured Russian tanks and armored personnel carriers to attack Russian positions.

Casualties have been sustained on both sides but figures are impossible to verify. Many civilians have also been killed or wounded.

Meanwhile, the Organization for Security and Cooperation in Europe (OSCE) has appealed for an immediate ceasefire in Chechnya and for resumption of negotiations on a peace settlement.

OSCE's chairman, Switzerland's Foreign Minister Flavio Cotti, issued a statement today saying the organization was "deeply concerned" at the heavy fighting in Grozny and the hundreds of casualties. Cotti said the new fighting was causing "unbearable suffering to the civilian population of Grozny."

The statement said the fighting violated the commitments made by both the Russian and Chechen separatist sides in the Moscow and Nasran agreements two months ago.